The Wilson Combat Tactical Single Stage trigger is modular trigger unit that can be easily installed in any AR pattern rifle by the end user in less than 1 minute without tools or even removing your selector. It is a ultra-crisp 5-5 3/4 lb. trigger pull that does not require any fine tuning or adjustment by the end user. It has no loose parts to misplace, the TTU is a totally self-contained trigger module. The hammer, trigger and disconnector are manufactured from virtually indestructible H13 steel and heat treated for maximum wear resistance. Wilson Combat AR-15 Tactical Trigger units will reliably function in any standard USGI style AR style lower receiver with standard GI trigger pins of .154" diameter regardless of caliber, including 7.62/.308 length AR receivers. Colt brand rifles made for commercial use between 1991 and 2009 will generally have a proprietary trigger pin size of .169" and are not compatible with any Wilson TTU. Colt rifles produced with pinned-in sear blocks are not compatible with any Wilson TTU. Wilson Combat TTU is not compatible with the Bushmaster ACR. The MR556 and any other AR style rifle with a firing pin safety mechanism in the bolt carrier is only compatible with the TTU 3G (TR-TTU-3G) or Two Stage Models (TR-TTU-M2). With .22 LR AR conversions or complete .22LR caliber AR-pattern rifles it is recommended that the standard single stage TTU (TR-TTU) or two stage TTU (TR-TTU-M2) trigger be used along with high or hyper-velocity ammunition for best reliability. The TTU is compatible with the S&W MP-10 .308 AR but the rifle's ambidextrous bolt catch must first be removed to install the trigger unit.